This invention relates to an apparatus for adding alloy material to a molten metal stream in combination with apparatus for maintaining an envelope of protective atmosphere about the molten metal stream.
When casting billets of steel, for example, it is known that the addition of aluminum to the molten metal as an alloying agent will facilitate production of a fine grained steel product. Heretofore aluminum has been added to the molten metal in the ladle prior to discharge from a nozzle or port into a mold. Mixing of aluminum with steel by addition in the ladle does not always provide a uniform or homogenous cast product. Moreover, the addition of aluminum to a ladle, particularly in larger amounts, may cause blockage of discharge nozzles or ports. Finally, as much as 70% of the aluminum may be lost in the slag.
Another method of adding aluminum to a cast product is exemplified by Tinnes in U.S. Pat. No. 3,459,346. There it is disclosed that a wire of alloying material may be fed by action of rolls through a tube into a molten metal stream as that stream is being discharged from a ladle into an ingot mold. A problem encountered with such a wire feeding arrangement is that a blockage may form in the tube through which the wire is fed. That is, the wire will often melt and clog the feed tube thereby rendering difficult the control and efficiency with which the alloying wire is fed into the molten metal stream.
Still another method of adding alloying materials to molten metal during a casting operation is suggested in the May, 1979 issue of Modern Casting Magazine at page 26. There it is disclosed that a nozzle may be used to direct small amounts of innoculent materials directly into the stream of molten metal flowing from a ladle into a mold. In the disclosure, fine granules of ferro silicon and other elements are added to molten iron as the iron is tapped from a holding furnace and directed into a mold. The apparatus for this innoculation procedure includes a flexible hose which receives a low pressure air supply and a supply of the innoculent granules. The materials are transported through the hose to a discharge nozzle which directs the material into the stream of molten metal.
Even though an alloying material may be successfully added to molten metal, a problem still often encountered in casting processes, including continuous casting processes, is contamination of the cast material by oxygen. Oxygen contamination adversely affects grain size, macrocleanliness, microcleanliness and the mold solidification process. In an effort to counteract the deleterious effects of oxygen contamination, various procedures have been proposed for casting in inert atmospheres. One practice calls for enclosing the total casting operation within a container and controlling the atmosphere within that container. Mahin, in U.S. Pat. No. 3,467,167, discloses such a process as does Sindelar et al in U.S. Pat. No. 3,262,073.
Alternative processes include the procedure disclosed by Holmes in U.S. Pat. No. 3,439,735 wherein a shroud is positioned about the molten metal stream as the stream flows from the ladle to a casting mold. Inert gas is fed into the shroud and retained by the shroud about the molten metal stream. Pollard, in U.S. Pat. No. 3,963,224 discloses a similar mechanism. Likewise, Hildebrandt, et al in U.S. Pat. No. 4,102,386 and Coward in U.S. Pat. No. 4,084,799 disclose similar shroud mechanisms.
In the continuous casting of steel, the addition of alloying agents as well as the protection of the molten metal stream from oxygen contamination have not heretofore been satisfactorily and simultaneously accomplished. The present invention contemplates a new method and apparatus which provides for the addition of alloying agents to a molten metal stream in a protective gas atmosphere.